Auxiliary power units (APUs) are gas turbine engines often used in aircraft systems to provide pneumatic and shaft power in addition to a main propulsion engine. As is known in the art, a compressor in the APU pumps air into a combustor at a given engine cycle maximum pressure (i.e., some multiple of ambient pressure) so that the combustor burns fuel in a high pressure environment. The burning fuel in the combustor heats the air prior to passing through the turbine. Because the products of combustion expanded through the turbine, above idle fuel flow rate develops more power than needed to drive the cycle compressor, some air (referred to as “bleed air”) can be drawn off and used as a pneumatic output for other aircraft devices. Excess shaft rotational speed on cold days causes the compressor efficiency to decrease because the cold air flow being pulled through the compressor makes the inlet air flow relative to the tips of the inlet blades in the compressor very fast, reaching velocities significantly higher than the speed of sound; that is, the compressor impeller blades see supersonic Mach numbers, driving the APU efficiency down.
Alternatively, the power can be used to drive a load compressor that compresses air in a separate stage, which is also driven by the powerhead components (i.e., compressor, combustor, and turbine stages).
Normally, the APU compressor and turbine are run at a constant mechanical shaft speed. Although a constant APU shaft speed keeps speed control simple, the ambient air inlet temperatures encountered by the APU can vary over a wide range during normal operation. At very low ambient temperatures, the APU compressor and turbine often experience lower efficiencies and narrower compressor flow ranges and consequent compressor/turbine matching problems. Extremely low temperatures also may cause excessive bleed air pressure. Although heavier and/or more complex, pneumatic systems may be designed to accommodate the additional pressure, this type of modification is cumbersome and expensive. Further, very high ambient temperatures adversely affect APU power and efficiency by diminishing air flow rate, bleed air pressure, and the engine cycle pressure ratio.
Running the APU shaft faster on hot days and slower on cold days can improve the compressor and turbine efficiencies, but these adjustments may also cause mechanical vibration and resonance in the APU, increasing the likelihood of failures in the blade, disc and shaft.
There is a desire for a method and system that can optimize APU operation in extreme ambient temperature conditions without causing vibration and resonance in the APU.